Quoting Andrew McGlashan (andrew.mcglashan@affinityvision.com.au):
I'm not positive, but I think if mail is in the queue and resolution for the domain name isn't forthcoming due to possible down DNS server(s), then the mail will stay in the queue and it will be tried for delivery later with fresh DNS requests as well (in terms of mail when it comes to DNS answers). Of course, finding a server for other services (web for instance) would be an immediate problem when the TTL expires.
Could be right. I'd not embarrass myself by saying your guess is right or wrong without checking. ;-> Being specific about scenarios, here: I'm pretty sure that any newly generated mail that triggers a DNS lookup that gets immediate socket failure on all auth nameservers is going to get immediate 45x hard fail. Hmm, a test mail to a non-existent FQDN is a logically identical case, wouldn't you agree? I could be missing something, otherwise, it would appear that in the scenario I describe the failure is immediate. (In my time zone, it's late-ish, and I probably shouldn't be trying to work through even gedankenexperiments. ;-> )